I've had the 645 for a long time. I added the 920 about a year ago. I have them set up with the same user info in different Garmin Connect accounts. I wear the 645 around my wrist and hold the 920 4 inches away around my palm. They are both paired with the same Wahoo HRM and so give me identical HR trackpoints. They both give me close to the same distance and time. But the 645 tells me my VO2max is 39 whereas the 920 says it's 34. Can anyone explain this?
